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    Home > Product > Power Systems > Power Transformer > Oil-immersed Transformer
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Oil-immersed Transformer

Oil-immersed transformers are a common type. Since transformers often operate under high-energy and high-temperature conditions, they are placed inside a steel tank filled with insulating oil. This oil provides both cooling and electrical insulation. Through natural convection, the oil circulates through and around the transformer core and windings, dissipating heat.
 

To prevent the degradation of transformer oil, its operating temperature should be kept below 85°C. For optimal performance and to minimize oil aging, the daily average operating temperature is typically maintained around 30°C.

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More About Oil-immersed Transformer

Types of Oil-immersed Transformers

  • Oil-immersed self-cooled (ONAN)
  • Oil-immersed air-blast / forced-air cooled (ONAF)
  • Oil-immersed water-cooled (OFWF or ONWF)
  • Forced oil circulation (OF or OD depending on flow direction)

Basic Structure of Oil-immersed Transformers

1. Active Part

  • Core
  • Windings
  • Insulation
  • Leads (including tap-changer, lead clamps, etc.)

2. Tank

  • Main Tank
  • Transformer Tank

3. Accessories

  • Conservator
  • Oil valves

4. Cooling System

  • Radiators
  • Fans
  • Oil pumps

5. Protective & Outlet Devices

  • Pressure relief device
  • Buchholz relay
  • Temperature sensors
  • Dehydrating breather
  • Bushings

Main Components

Advantages of Oil-Immersed Transformers

  • Lower cost – Oil-filled units are significantly more economical, often costing about half as much as other types of similar capacity.
  • Superior cooling performance – Oil serves as an efficient cooling medium, providing better heat dissipation.
  • High voltage capability – Oil-immersed designs can operate at very high voltage levels without the limitations found in some other transformer types.
  • Versatile applications – They can serve effectively as both power transformers and distribution transformers.
  • Environmentally friendlier in end-of-life – They are easier to recycle compared to many alternatives.
  • Compact size – Their design allows for a relatively small footprint.
  • Suitable for outdoor installation – They are best deployed outdoors, where any potential oil spill or leakage poses less risk.

Application Scenarios of Oil-Immersed Transformers

  • Power transmission and distribution networks – Widely used in substations and grid systems for stepping up or stepping down voltage levels.
  • Industrial facilities – Suitable for factories, mines, and manufacturing plants that require high voltage capacity and reliable power supply.
  • Power plants – Employed in thermal, hydroelectric, and nuclear power stations to connect generator output to transmission lines.
  • Outdoor substations – Ideal for outdoor installation due to robust construction and effective cooling.
  • Renewable energy systems – Used in large-scale solar and wind farms to collect and step up generated power for grid integration.
  • Railway power systems – Applied in traction substations to supply suitable voltage for electric trains.
  • Commercial and residential areas – Serves as distribution transformers to deliver power to buildings and housing complexes (typically installed outdoors on pads or poles).
  • High-voltage applications – Preferred where operating voltages exceed 35 kV, such as transmission-level networks.
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